So there's two numbers at play here: balance sheets and market cap.
A company's market cap is its share price multiplied by the number of outstanding shares. Using parent's Netflix example, there are approximately 450m Netflix shares, times a market price of $450, for a market cap of approximately USD 200b. So it's fair to say that Netflix, right now, is worth some $200b.
Now the market cap can sometimes be misleading: Say you start Runaway Co and issue 100m shares. Each of these shares is worth what the market is willing to pay for it. If I agree to buy one off of you for $100, then we've just spent $100 to make a $10bn company. That's where the trade volume comes in.
Netflix is a publicly traded company. Today, some 7.5m Netflix shares were traded at prices ranging from $443 and $456. Given that volume, you can be relatively sure that the market price accurately reflects the company's current value (as seen by the market).
On the other hand, a company's balance sheet is listing its assets and liabilities. That's where the huge numbers come in for banks. Let's say a bank receives $1m of deposits from its customers, and uses that money to fund a $1m mortgage. That bank's balance sheet is now $1m.
Of course, that doesn't necessarily mean that the bank is now worth $1m -- all of the $1m in mortgage debt that it is owed is offset by $1m in deposits that it owes to its depositors. Instead, the bank would be valued according to the profit it can generate from this. If it charges 2% for the mortgage and pays 1% interest to its depositors, that leaves 1%, or $10k per year. Valued at 10X profit, that bank might be worth $100k or so, even though its balance sheet lists a far bigger number.
Which takes us full circle.
As you said, Citi has some 2tn assets on its balance sheet. In 2019, it generated some $75bn in revenue from this, and made a profit of some $20m. At approximately $45 per share, its market cap is right around $95b.
Netflix, on the other hand, had about $33b on its balance sheet. Way fewer assets, but also way fewer obligations. In 2019, it generated revenues of about $20bn and some $4bn in profits. Investors appear to believe that there is a lot of growth still ahead, because they value the company at $200b.